1. Field of the Invention
The present invention relates to a pulse frequency modulation (PFM) control circuit and, more particularly, to a PFM control circuit for converting voltages with high efficiency over broad loading requirements.
2. Description of the Related Art
For providing electronics devices with a stable power supply so as to ensure steady performance, voltage regulators are indispensable components. The voltage regulator supplies a required amount of current to a load at a regulated output voltage. Therefore, when the electronics devices are used as the loads with respect to the voltage regulators, the performance of the electronics devices is kept stable and prevented from being affected by fluctuations of the power supply. In addition to the regulation of the power supply, the lifetime of the power supply is also an important issue to be considered for a variety of portable electronics devices powered by isolated and finite sources such as batteries.
In terms of the efficiency of voltage conversion, linear voltage regulators due to power loss caused by passive regulating transistors are inferior to switching voltage regulators. The switching voltage regulator typically includes a control circuit for turning on and off a switching circuit. Through appropriately control the duty ratio of the switching circuit, i.e. a ratio of an ON-time of the switching circuit to a period of a switch cycle, the output voltage is regulated to a desired level for supplying current required by the load. In prior art are there many well-known control circuits and methods, such as a pulse width modulation (PWM) mode, a constant ON-time PFM mode, and a constant OFF-time PFM mode. However, the conventional control circuits and methods achieve a high efficiency of voltage conversion only within a limited range of loading requirements. For example, the PWM mode provides a preferred efficiency in a heavy loading operation, the constant ON-time PFM mode in a light loading operation, and the constant OFF-time PFM mode in a heavy loading operation. In order to convert voltages with high efficiency both in heavy and light loading operations, it has been suggested that an automatic selection be made between the PWM mode and the PFM mode depending on the magnitude of the loading requirement. However, such a solution must combine two different kinds of control circuits into the same integrated circuit chip as well as a specially-designed mode selection circuit, resulting in complicated circuitry and a higher cost.
Therefore, it is desirable to develop a control circuit and method of converting voltages with high efficiency over broad loading requirements.